This invention relates to supercharged internal combustion engines.
The practice of supercharging internal combustion engines by means of turbochargers to improve their power output is well known in the art.
Each turbocharger normally comprises a turbine rotor and a compressor rotor mounted coaxially on the same shaft, the turbine being driven by the energy present in the exhaust gases from the reciprocating internal combustion engine and the compressor serving to compress air for delivery to the engine.
In one particular type of turbocharger, employed in an embodiment of the invention to be described later, there are three major functional sections. The central section consists of a housing which carries the bearing system for supporting a rotor consisting of a shaft with, at one end a centrifugal compressor impeller, and at the other end the rotor of a radial-flow turbine. The compressor end of the bearing housing carries a circular flange to which is attached the compressor casing of the centrifugal compressor.
At the opposite end of the bearing housing there is associated with the turbine rotor a turbine casing into which exhaust gas is conducted from the engine. The casing serves to shape the flow of the hot gases before their expansion through the turbine rotor.
The gases entering the turbine casing are generally hot, with temperatures up to 700.degree. C. not being uncommon, and it is therefore an advantage to the bearings if the turbine casing can be supported as independently as possible of the bearing housing, thereby to reduce heat flow from the turbine casing into the bearing housing.
The bearing housing requires a supply of cool oil to lubricate the bearing supporting the rotor and a drain means to take the used oil away.
A conventional method of support for turbochargers is to support a turbocharger via a flange attached to the inlet of the turbine casing. Whilst this may be simple it means that a considerable dead load is applied to the exhaust ducting which conveys the exhaust gases from the engine to the turbocharger.
It is also known to support turbochargers via a flange attached to the outlet of the turbine housing, which gives rise to similar problem regarding deadload. It is also known to support turbochargers by a support-bracket coupled to the central section.
For single-stage turbocharging it may be sufficient to use a single supercharger. For larger engines, in particular those having two rows of cylinders, each row of cylinders may be provided with its own supercharger. An example of such an arrangement is disclosed in United Kingdom Patent GB437078.
The pressure ratio obtainable by a single stage turbocharger is somewhat limited, and for higher performance, multiple stage turbocharging has been employed, the required degree of compression being provided by disposing a high pressure turbocharger in series with a low pressure turbocharger. Exhaust gas from the engine first passes through the turbine of the high pressure turbocharger and then passes through the turbine of the low pressure turbocharger. Similarly, air at atmospheric pressure is first compressed in the low pressure compressor and is then further compressed in the high pressure compressor.
However, this does give rise to problems. The turbines get very hot in use and as a consequence their turbine casings and associated exhaust duct work are subject to thermal expansion when starting from cold. This gives rise to problems in maintaining the exhaust duct work gas-tight. This can be a problem where engines are installed in confined spaces such as ships or basements of buildings.
To overcome the problems of exhaust gas leakage been proposed to construct a two-stage turbocharger with a unitary combined casing for the high and low pressure turbines. Such arrangements are disclosed in U.S. Pat. No. 4,032,262 (Zehnder), and U.S. Pat. No. 4,196,593 (Froeliger) and involve the provision of a unitary casing for the high and low pressure turbocharger.
Where two-stage turbocharging is applied to large engines, the low pressure turbocharger may prove to be physically large such that mounting on the engine is not practical. The low pressure turbocharger then has to be mounted off the engine and coupled thereto by ducting. Such an arrangement is disclosed in the publication "Shipbuilding and Marine Engineering International" April 1977, page 171 and the article "Development of two-stage turbocharging system on a four stroke medium speed diesel engine" CIMAC, 12th International Congress on Combustion Engine, Tokyo 1977, paper A7 FIG. 16. Such arrangements require considerable headroom and present difficulties where an engine is to be installed in a confined space having limited headroom.
It is sometimes the practice not to cool the turbine casings, inter alia because condensation of combustion products on a cooled turbine casing has been found to lead to corrosion and consequent reduction in service life. This has exacerbated the problems of thermal expansion. Further, the elimination of cooling from the turbine housing leads to increased heat loss from the engine. This can be a problem in confined spaces.
An alternative method of supporting the turbocharger is known from U.S. Pat. No. 4,400,945 (Deutschmann).
U.S. Pat. No. 4,400,945 (Deutschmann) provides a housing to which turbochargers are mounted such that the turbines and their associated conduits are disposed within the housing, while the compressors and their associated conduits are outside the housing. The housing may be sealed to prevent escape of any exhaust gas which might leak pass the joints in the exhaust conduits.
Deutschmann teaches to arrange the bearing housing to be supported by a box-like enclosure surrounding the turbine casing. The enclosure is split along a plane passing through the centre-line of the bearing housing, and the two halves of the enclosure are arranged to tighten down onto the bearing housing, this being achieved by arranging the bearing housing to be of cylindrical shape, and the wall of the enclosure to have an accommodating circular hole equally displaced about the split-line. If the enclosure's grip on the turbocharger is to be secure and stable, then the bearing housing diameter and the internal diameter of the hole in the enclosure have to be manufactured to fine limits. With this arrangement, the turbocharger can only be removed by dismantling the enclosure to some degree, and where a sealed housing is employed, the joints between the two halves of the housing along the dividing plane and around the turbochargers have to be re-sealed, and the connections between the top of the housing and any other components, such as the exhaust pipe, which had to be disconnected before the top of the housing could be removed, must be re-connected and re-sealed.
Deutschmann also mitigates the problem of headroom by provision of a relatively large number of individual relatively small turbochargers or two-stage turbocharger assemblies rather than one or two relatively large turbochargers. However, the low pressure turbochargers Deutschmann's two-stage turbochargers are physically larger than their associated high pressure turbochargers.